can't login through desktop GUI after upgrade from ubuntu 10.04 to 12.01 desktop

I have just upgraded ubuntu 10.04 32 bits desktop to ubuntu 12.04 32 bits desktop (through login into ubuntu 10.04 desktop GUI screen, and then upgrade to 12.01 through the popup "update manager" GUI screen), but got can't login though GUI screen problem, i.e. when login through the GUI screen with the correct password, after a few seconds, it is back to the same GUI login screen again. However, it can login in through terminal.

Have spent sometime on googling and trying to resolve the problem, but unsucessful (during this period of time, run command tasksel several times to install and remove some applications, and I think this may cause some config data of the system and some applications removed, but I'm not quite sure whether it is the case). I have previously had (when under ubuntu 10.04) a lot of very important applications, data/files installed already (under my own user directory, i.e. /home/qiubosu/), e.g. wine, openoffice, virtualbox (and the vm OS images) etc applications. To avoid to damage/destroy these applications, data/files, I think it isn't good to carry on "fixing" the problem in the same approach.

it is much appreciated if anyone can help and give some advice on how to resovle this problem.

I have below several different approaches, but not quite sure whether they are good enough and won't have harmful impact to the system and the installed applications, data/files:

1) burn a CD with the new version of ubuntu desktop, i.e. 12.10, and do a reinstall from the CD to upgrade from the current ubuntu 12.01 to new version of ubuntu, in order to fix some current system problems, i.e. missing some system config data etc.

can anyone make comment on whether this approach is safe and can do the job?

2) to protect the above mentioned important applications, data/files already installed, use an empty harddrive (as 1st harddrive) to do a full install of the ubuntu 12.01 desktop with a burned CD, use the current harddrive with the mentioned applications, data/files, as a 2nd harddrive. when do a full install of ubuntu 12.01 on the 1st harddrive, is it possible to add the 2nd harddrive (the current harddrive with the mentioned applications, data/files installed) as part of the partition of the new ubuntu installation, so these applications, data/files can be accessed under the new ubuntu 12.01 desktop without any problems? if yes, how to achieve this?

3) apart from the above approaches, are there any better approaches to have a well-working ubuntu desktop OS (of course, can login through desktop GUI screen, and this is the reason for this post), and can access the mentioned applications, data/files from there without any problems?